Motion training aid with stimulator

ABSTRACT

A training aid stimulator for providing fast perceptive feedback is disclosed. The training aid stimulator includes a first skin electrode and a second skin electrode both for making electrical contact to the body of a user, a charging module, a discharge module connected to one or more of the skin electrodes for a feedback discharging, and a processor for controlling the charging of a capacitor equivalent to a predetermined first voltage level, wherein the processor further being connected to the discharge module for controlling a feedback discharge of the capacitor equivalent. The stimulator comprises a voltage measurement module for measuring the level of charge of the capacitor equivalent, and the processor is configured for keeping the stimulator ready to discharge by repeatedly measuring the level of charge and by providing a maintenance charging when the voltage over said capacitance equivalent is at or below a predetermined second voltage level.

RELATED APPLICATIONS

This application is a continuation of pending U.S. patent applicationSer. No. 16/790,419 filed on Feb. 13, 2020, which is a non-provisionalapplication under 35 USC 111(a), claiming priority to InternationalSerial No. PCT/EP2017/080677, filed on Nov. 28, 2017, the entirety ofwhich both are herein incorporated by reference.

TECHNICAL FIELD

The present invention relates to the field of training aids, i.e.,devices that helps a person or animal better perform some activity ofthat person or animal. More particularly the present invention relatesto motion training aids, i.e., systems or devices that provide some kindof feedback relating to a motion performed by the user. Even moreparticularly it relates to sports movements training aids.

PRIOR ART

One example of motion training aid is known from WO2003024544. Itdiscloses a repetitive motion feedback system provided with varioussensors and devices for monitoring aspects of a repetitive motionsequence, such as a golf swing. The monitored aspects can include motionproperties of an object moved by the user, position properties of theuser and motion properties of the user. A data processing system forreceiving data of the monitored aspects provides feedback data that isprovided to a feedback output device, such as a graphical display deviceor speaker, so that the user is provided with feedback regarding therepetitive motion sequence. In one particular embodiment, the user'sperformance is compared to a template of a prior performance, withfeedback being provided regarding the differences.

Another prior art document is U.S. Pat. No. 6,778,866 disclosing amethod and apparatus for teaching a person how to perform a specificbody motion in a consistent manner is based on electronically measuringone or more parameters of an actual body motion, comparing the one ormore measured parameters with corresponding parameters of a target bodymotion, and providing a sensible feedback to the user based on a degreeof correspondence between the one or more measured parameters and thecorresponding target parameters. In a particular embodiment, thefeedback is audible. More specifically the feedback is a musical tunethat has a particular characteristic (such as rhythm) that isparticularly suited to a particular body motion (such as a golf swing).The feedback may be in the form of electronically causing the musicaltune to go off-key in proportion to a discrepancy between the actualbody motion and the target body motion.

A further prior art system and method for teaching ergonomic motion ofan athlete, for example a golfer is disclosed in WO200518759. The systemincluding the video camera for capturing successive image of the golferexecuting a preferring golf swing and a threshold definition system thatallows the golfer define a spatial region of the video image. If thespatial region is intruded upon, an alarm is actuated, thereby providingfeedback so the golfer may alter the technique of the next attemptedmotion. For example, the golfer may define the region such that if theclub moves off plane during a swing, a tee removal system causes theball to disappear. In this manner, the golfer is only able to hit theball when the club stays on plane.

SUMMARY OF THE INVENTION

The inventors are aiming at providing a device for providing fastfeedback for training devices used to train and perfect some kind ofuser behaviour, or action, wherein this behaviour or action is happeningrelatively quickly. Examples of such behaviours or actions include butare not limited to e.g. sports motions, such as technically complexmotions occurring in e.g. athletic field events (high jump, pole vault,hammer throw, javelin throw etc), or gymnastics (jumps, choreography,cheerleading moves), or baseball, or golf (golf swing, putting stroke),just to mention a few.

The inventors have devised that it is desirable that a feedback signalsignalling a less efficient move, or a “bad” move, or a movement thatdeviates from a reference movement should be instantaneous, or at leastperceived as instantaneous by the user. The stimulus unit of anembodiment of the present invention is configured to deliver thestimulus with very short delay, preferably, less than 50 ms, or morepreferred less than 20 ms, or most preferred less than 10 ms. Thestimulus should also be distinct. The inventors are aiming at providinga noticeable and distinct feedback in order to indicate and point out anundesirable motion to discourage said undesirable motion by the user.

In various embodiments, the stimulus unit comprises a charging moduleand a discharge module and body electrodes. The charging module isconfigured to charge a capacitor equivalent that may comprise a “bodycapacitance” inherent to the body of a person, or a charge holdingcapacitor. The charging module is configured to charge the capacitorequivalent to a charging level that is sufficient to cause, atdischarge, a sensory sensation, i.e., an electric shock sensation orsimilar sensation to the person or animal performing the activity. Thedischarge is supposed to be triggered when there is detected adisagreement between the position values of a current motion and of apredetermined desired motion. The level of charge or the voltage of thecapacitor equivalent is measured and further charge is supplied whennecessary to maintain a level that is sufficient to provide the electricshock or similar sensation.

Thus, in alternate embodiments different couplings may be provided toutilize separate capacitor couplings or utilizing the inherent bodycapacitance of a user. There is also disclosed electrical couplings toprovide an efficient stimulus signal to the user, as well as the signalitself.

The stimulator unit may further be provided with a particular circuit toconnect and disconnect a measurement module so as to save energy by notletting electrical charge leak out through the measurement module.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the manner in which the above recited and other advantagesand objects of the invention are obtained will be readily understood, amore particular description of the invention briefly described abovewill be rendered by reference to specific embodiments thereof which areillustrated in the appended drawings.

Understanding that these drawings depict only typical embodiments of theinvention and are not therefore to be considered to be limiting of itsscope, the invention will be described and explained with additionalspecificity and detail through the use of the accompanying drawings inwhich:

FIG. 1a shows a block diagram of a stimulator device according to anembodiment of the present invention.

FIG. 1b shows a block diagram of a stimulator device according to afurther embodiment of the present invention.

FIG. 1c shows a block diagram of a stimulator device according to astill further embodiment of the present invention.

FIG. 2a shows a principal circuit diagram of a first basic embodiment ofa stimulator device.

FIG. 2b shows a principal circuit diagram of a second basic embodimentof a stimulator device.

FIG. 3a shows a more detailed principal circuit diagram of thestimulator device of FIG. 2 a.

FIG. 3b shows a more detailed principal circuit diagram of thestimulator device of FIG. 2 b.

FIG. 4 shows a flowchart over method steps for providing stimulationfeedback.

FIG. 5 is a diagram showing voltage as a function of time during acharging process used in a stimulator device according to an embodimentof the present invention.

DETAILED DESCRIPTION Definitions

For the purpose of the present invention, and in the following text, thefollowing terms are used with the meaning as explained below.

“Motion representation”: A “motion representation” is a usuallymathematical representation of a motion. The motion representation mayinclude representations of linear and rotational motion position, motionvelocity, and motion acceleration. For example, the motion may berepresented by the current position of a predetermined point on the bodyof a user, or the motion may be represented by a (motion) track, seebelow.

“Position”: With the term “position”, as used herein is understood thephysical local position of a sensor unit or small object in relation toa nearby reference point, and expressed using a suitable coordinatesystem. Typically, in the context of the present invention, positionsare within the magnitude of 0-5 meters from the reference point.

“Undesired motion”: The term “undesired motion” is used to denote amotion that is undesired or comprises an undesired feature as seen fromthe point of view of the user, and/or his or her coach.

“Body motion tracker”: As used herein, the term “body motion tracker”denotes a device or a system, or a piece of computer code when executedcapable of tracking one or more predefined points of a user's body overtime, based on processed sensor data.

“Tracking”: With the term “tracking” is understood the activity ofcollecting and storing (recording) consecutive positions of one or morepredefined points on a user's body during a motion.

“Motion track”: With the term “motion track” is meant the result of thetracking activity, i.e., the collective amount of stored consecutivepositions of a predefined body point over time, starting at a startpoint or start time, and ending at a finishing point or finishing time.

“Reference motion track”: A “reference motion track” is a desired motiontrack that can be used to create a model to which motion representationsof motions can be compared.

“Rotation angle” or “Angle of rotation”: In two dimensional space the“angle of rotation” is a measurement of the amount, the angle, by whichan object is rotated about a fixed point. In three-dimensional spacerotation is measured and indicated using angles of rotation about threecoordinate axes.

“Predefined body point”: With the term “predefined body point” is meanta point on a user's body that has been provided with means forfacilitating the tracking of said point, e.g. a sensor unit.

“Attitude”: In the context of the present invention the term “attitude”is used to denote an object's orientation (attitude, angular position)in space. The attitude may be represented by pitch, yaw and roll anglesor, alternatively, by an attitude vector or axis, and a rotation anglearound that vector or axis, i.e. axis-angle representation, c.f. Euler'srotation theorem.

“Motion sensor unit”: A “motion sensor unit” is understood to be a unit,attachable to a user's body, that are able to deliver motioninformation, such as accelerations, information making it possible todetermine the sensor's attitude and three-dimensional position orchanges in the same position during a motion of the user, in a suitablereference system. The sensor unit is conceived to be small andlightweight enough not to interfere with the motion of the user.

“Control unit”: In the context of the present invention a “control unit”is a unit comprising a man-machine interface for operating a device, italso usually comprises wireless communication means to communicate withthe processor and/or the motion sensor unit.

“Sample”: In the context of the present invention the term “sample” isused to denote a calculated state of the motion sensor unit at aparticular moment in time, and may include representations of linearand/or rotational: motion position, motion velocity, and motionacceleration as calculated by the processor based on motion sensor datafrom the motion sensor unit and also based on a reference frame, i.e., acoordinate system. Associated with the sample is a sample number and/ora sample time.

“Processor”: In the context of the present invention the term“processor” is used to denote a processor system irrespective if itcomprises one or more logical or physical processors, if nothing else isexplicitly mentioned.

“Memory”: In the context of the present invention the term “memory” isused to denote a memory system irrespective if it comprises one or morelogical or physical memories, if nothing else is explicitly mentioned

“Stimulator”: In the context of the present invention the termstimulator is used to denote a device, attachable to the body of aperson or animal, and upon receiving a command, capable of eliciting astimulus perceptible by that person or animal.

“Motion”: With the term “motion” is understood any body movementperformed by a person, composite or simple, may it be a movement of oneor more of his or her extremities, or torso, or centre of gravity. Anypossible ambiguities should be solved by the context in which the termis used. The term is also used to denote the movement as sensed by asensor. Example motions include portions of or complete high jump, polevault, hammer throw, javelin throw, gymnastics jumps, choreographymoves, cheerleading moves, baseball batting, baseball pitching, golfswing, putting stroke.

The stimulus unit is configured to deliver the stimulus with very shortdelay, preferably, less than 50 ms, or more preferred less than 20 ms,or most preferred less than 10 ms. The stimulus should also be distinct.

In order to be able to deliver the stimulus, i.e., an electricdischarge, with short notice, there is provided a training aidstimulator device as outlined in FIG. 1a , The block diagram in FIG. 15shows a stimulator device according to an embodiment of the presentinvention. A processor 110 is connected to a memory 120. Further, theprocessor is connected to a charging module 125 for instructing thecharging module 125 to charge. The charging module 125 is connected to afirst body electrode 135 to provide charge to charging a bodycapacitance connected via the first body electrode 135. The chargingmodule 125 may be connected to the first body electrode via an impedancemeasurement module 155, this will be discussed later.

Further the processor 110 is connected to a measurement switch 130. Themeasurement switch is controlled by the processor to connect anddisconnect a voltage measurement module 150 to the first body electrode135 at certain times. The timing of the connecting and disconnectingrespectively of the measurement module to the first body electrode 135will be further explained below. The processor 110 is further connectedto a discharge module 160 in order to control the delivery of adischarge of the body capacitance connected via the first body electrode135 and a second body electrode 137. The discharge switch module 160 isconfigured to connect the body second electrode 137 to ground uponcommanded so from the processor 110, to effectively cause a dischargethrough the skin of a subject using the stimulator.

The block diagram in FIG. 1b shows a stimulator device according tosecond embodiment of the present invention, A processor 110 is connectedto a memory 120. Further, the processor is connected to a capacitorcharging module 125 for charging a charge holding capacitor 139, C1. Afirst electrode of the charge holding capacitor is connected to thecapacitor charging module 125. A second electrode of the charge holdingcapacitor 139, C1 is connected to ground. Further, the processor 110 isconnected to a measurement switch 130. The measurement switch iscontrolled by the processor to connect and disconnect a voltagemeasurement module 150 to the first electrode of the charge holdingcapacitor 139, C1 at certain times. C1 is dimensioned to be able to holda charge sufficient to be able to cause a clearly perceivable feedbacksensation with the user, when C1 is discharged.

The timing of the connecting and disconnecting respectively of thevoltage measurement module 150 to the first electrode of the chargeholding capacitor 139 will be further explained below.

The processor 110 is further connected to a discharge module 161 inorder to control the delivery of a discharge of the charge holdingcapacitor 139 connected via a second body electrode 137, the skin 140 ofa user and a first body electrode 135. The discharge module 161 isarranged to connect the body second electrode to the first electrode ofthe charge holding capacitor 139, possibly via an impedance measurementmodule 155, upon commanded so from the processor 110. The first bodyelectrode 135 in this embodiment preferably connected to ground.

An advantage of this coupling is that charge deposited in the chargeholding capacitor 139 can be kept isolated from the body untildischarge, and thus isolated from a risk of possible electrical leakageassociated with biological tissue.

FIG. 1c will be described later in context of impedance measurements.

FIGS. 2a, and 3a show alternate principal wiring diagrams for a firstcircuit, and a second circuit capable of providing a distinctbiofeedback electrical stimulus to the skin of a user. FIG. 2a shows astimulator circuit comprising a first electrode 135 and a secondelectrode 137 devised to make contact to the skin of a user. Said firstand said second electrodes can also be referred to as skin electrodes.Power at a low voltage level is supplied at V_(SUPPLY). The low voltagenormally is DC (Direct Current) supplied by a battery which is steppedup in a transformer device 138 to a higher level, V_(IN). In variousembodiments, the transformer device 138 comprises a DC-to-DC converteror an electric power converter. A diode 199 is provided after thetransformer device 138 and connected to connection point V_(IN). Ameasuring switch 130 is connected between V_(IN) and a first resistor R₁of a voltage divider. The voltage divider is made up of the firstresistor R₁ and a second resistor R₂. An analogue to digital converterADC is connected between the resistors R₁ and R₂ and could be said toconstitute a voltage measurement module 150 together with theseresistors. The measuring switch 130 is connected to and controlled by asignal MEASURE from the processor 110. An electrical model for thestimulator circuit as attached to the human skin has been devised. Themodel includes a capacitance of the body, C_(BODY), and a resistanceR_(BODY) of the skin between said first electrode 135 and said secondelectrode 137.

The stimulator circuit further comprises a discharge switch 161 forcontrolling a feedback discharge also called a biofeedback electricalstimulus. The discharge switch 161 is connected between the second skinelectrode 137 and earth to provide a discharge drain when triggered by atrigger signal from trigger output, TRG, of the processor. When avoltage is applied at V_(SUPPLY) and increased in the transformer device138 the capacitance of the body, C_(BODY), will be charged to apredetermined level. The actual level is measured by applying the signalMEASURE and reading a voltage signal in the voltage divider at ADC. Ananalogue to digital converter can be used to provide the processor 110with the actual reading of voltage level. When the predetermined levelis reached the processor 110 is ready to provide a signal at TRG to openthe discharge switch 161. As a result, the capacitance C_(BODY) isdischarged and a pulse is experienced by the person carrying said firstelectrode 135 and said second electrode 137.

The TRG signal is delayed until a specific condition is present. Acontinuous discharge through first resistor R₁ and second resistor R₂ isavoided in the embodiment shown in FIG. 2a by opening the measuringswitch 130. In the embodiment shown in FIG. 3a an implementation of themeasuring switch 130 comprises a first transistor 165 connected at acontrol input to a first control resistor 167 receiving the MEASUREsignal. In a similar way, the discharge switch 161 comprises a secondtransistor 168 connected at a control input to a second control resistor169 provided for receiving the TRG signal from the processor. Still,there will be some minor continuous discharging from C_(BODY), but by amaintenance or support charging as described below with reference toFIG. 4 and FIG. 5 this problem can be avoided.

The process of maintenance charging (support charging or top-upcharging) is started in block 205 and the charging is started in block210. The processor 110 then applies signal MEASURE to the measuringswitch and the voltage over second resistor R₂ is measured in block 215.In block 220 it is checked whether a predefined first voltage level isreached. If the predefined first voltage level is not reached thecharging in block 210 continues. If the predefined first voltage levelis reached the processor continuously awaits a condition for a feedbackdischarge in block 225, When a condition for discharge is at hand thedischarge module 160 is activated in block 230. The device then isstopped in block 232. If there is no condition for discharge theprocessor enters a wait condition for a predetermined time period inblock 235.

The processor then checks if the full process is finished in block 240.If the process is finished, the charge of the body capacitance is slowlydissipated, and the device then is stopped in block 232. If the processis still in operation (motion continues) the voltage measurement module150 is activated in block 242 and it is checked in block 245 whethervoltage of voltage divider R₁, R₂ has dropped to or below apredetermined second level, c.f. FIG. 5. If voltage has not dropped toor below said predetermined second level the measuring module isswitched off in block 246 and the process returns to block 235.

If voltage has dropped to or below said predetermined second level themeasuring module is switched off in block 248 to preserve charge. Thenmaintenance charging is activated in block 250. The maintenance chargingis maintained for a predetermined time period after which the voltagemeasurement module 150 is activated in block 252, In block 254 it ischecked whether the predefined first voltage level is reached. If thepredefined first voltage level is not reached the measuring module isdisconnected in block 248 and the maintenance charging in block 250continues. If the predefined first voltage level is reached themeasuring module is disconnected in block 246 and the process returns toblock 235.

FIGS. 2b and 3b shows corresponding principal wiring diagrams forembodiments wherein a discharge switch module 161 is arrangedelectrically close to the first electrode 135, thereby effectivelyisolating the body from the charging voltage and a charge holdingcapacitor 139, C1.

The diagram of FIG. 5 shows a diagram of voltage vs time during thecharging and maintenance charging processes described above withreference to FIG. 4. A main charge process corresponding to the chargeblock 210 is initiated at time T1. When the predetermined first voltagelevel is reached, a discharge process starts. The dotted line indicatesthe discharge that would take place, should the measurement switch 130be omitted. The voltage level in that case soon would reach a levelwhere a controlled discharge would not be sensed by the user in theintended way. As a result of the voltage measurement module 150measuring a low voltage level, a support or maintenance charging insteadwill be initiated when the voltage level drops to or below the secondlevel. As soon as the first voltage level is reached again the chargingis stopped. The voltage drop between the first level and the secondlevel is at a level that will not impair the feedback discharge. Thesupport or maintenance charging will be initiated repeatedly when thevoltage level reaches the second level.

Charging Module

The internal structure of the charging module 125 and the capacitorcharging module 125 may preferably include a voltage transformationcircuit to step up a battery voltage of a handful of volts to a suitablecharging voltage of maybe up to a couple of hundred volts.

Stimulating Signal Structure

The stimulator is configured to provide the stimulating signal as apulse train of one or more pulses. The processor and/or the dischargeswitch module 160, 161 may be configured to release the discharge in aone pulse fashion only. However, inventors have surprisingly found thata stimulus signal of two to four pulses of 1-10 milliseconds length and5-20 milliseconds periods of zero voltage in between may generate asensation suitable for its purpose. The stimulator thus may beconfigured to provide a discharge voltage preferably in the interval of2 V to 450 V, and more preferred in the interval of 200 to 450 V. Thestimulator may be configured to provide a stimulus signal that does notviolate statutory requirements for medical equipment.

Body Impedance Measurements

The stimulator may further be provided with a body impedance measurementmodule to measure body impedance between the two skin electrodes 135,137. The body impedance measurement module 155 being coupled to theprocessor and also to the first and second body electrodes or to oneelectrode and to ground. Preferred topological arrangements are shown inFIGS. 1a, 1b and 1 c.

FIG. 1a shows a block diagram of a stimulator where an impedancemeasurement module 155 arranged electrically between the charging module125 and the body first electrode 135. The processor is configured tocontrol the charging module 125 to provide a pulsed voltage that can beused for measuring. The processor is preferably configured to controlthe discharge switch module 160 connected to ground to allow current toflow in this case.

FIG. 1b shows a block diagram of a stimulator where an impedancemeasurement module 155 is arranged electrically between the dischargemodule 161 and a body electrode 137. The device is configured such thatcharging module provides a low voltage, typically around 5 Volts, toallow the impedance measurement module to measure impedance. The deviceis configured such that the discharge module connects the chargingmodule and the impedance measurement module to allow current to flow.

FIG. 1c shows a block diagram of a stimulator where an impedancemeasurement module 155 is arranged electrically between the chargingmodule 125 and the discharge module 161. In this case the stimulatordevice is configured to let the discharge switch module 161 connect theimpedance measurement module 155 to the electrode 137 and current flowsfrom the charging module and the charge holding capacitor 139 throughthe switch and to the body electrode 137 through the body skin portionand to the electrode 135 and to ground.

The processor is preferably configured to adapt the voltage of thestimulus signal based on measurements of the body impedance from thebody impedance measurement module.

For example, a higher impedance measurement may be used by the processorto direct the capacitor charging module 125 to provide a higher voltageto produce a stimulus current equivalent to a case of lower bodyimpedance and lower voltage. The body impedance measurement module maybe realised as a separate module or being realised using the voltagemeasurement module 150 and providing a measurement signal via thecapacitor charging module. The capacitor charging module 125 in suchcase being configured to provide such a measurement signal.

While certain illustrative embodiments of the invention have beendescribed in particularity, it will be understood that various othermodifications will be readily apparent to those skilled in the artwithout departing from the scope and spirit of the invention.Accordingly, it is not intended that the scope of the claims appendedhereto be limited to the description set forth herein but rather thatthe claims be construed as encompassing all equivalents of the presentinvention which are apparent to those skilled in the art to which theinvention pertains.

LEGEND

-   100 Stimulator-   110 Processor-   120 Memory-   125 Charging module-   130 Measurement switch-   135 First skin electrode-   137 Second skin electrode-   138 Transformer device-   139, C1 Charge holding capacitor-   140 Body skin portion-   150 Voltage measurement module-   152 Analog to digital converter-   155 Impedance measurement module-   160 Discharge module-   161 Discharge switch module-   165 First transistor (Measurement switch)-   167 First control resistor (Measurement switch)-   168 Second transistor (Discharge switch)-   169 Second control resistor (Discharge switch)-   199 Diode-   C0 Capacitor-   R1, R2 etc Resistances

The invention claimed is:
 1. A training aid stimulator comprising: afirst skin electrode for making electrical contact to a body of a user;a second skin electrode for making electrical contact to the body of theuser; a charging module connected to a capacitor equivalent; a dischargeswitch module for controlling a feedback discharging of an electriccharge; a processor connected to the charging module for controlling thecharging of the capacitor equivalent to a predetermined first voltagelevel, the processor further being connected to the discharge switchmodule for controlling the feedback discharging of the electric chargethrough the first skin electrode and the second skin electrode; avoltage measurement module for measuring a level of charge of thecapacitor equivalent; and a body impedance measurement module configuredto measure a body resistance of the user, wherein the processor isconfigured for keeping the training aid stimulator ready to discharge byrepeatedly measuring the level of charge of the capacitance equivalent,and by providing a maintenance charging by activating the chargingmodule when a voltage of the capacitor equivalent, is at or below apredetermined second voltage level, wherein the training aid stimulatorbeing configured to: disconnect the voltage measurement module when afirst voltage level is reached; wait a predetermined period of time; andre-connect the voltage measurement module and provide maintenancecharging if a measured voltage level is at or below a second voltagelevel, and wherein the processor is configured to adapt a voltage of astimulus signal based on the measured body resistance.
 2. The trainingaid stimulator of claim 1, wherein the training aid stimulator isconfigured to handle the capacitor equivalent as if the capacitorequivalent is constituted to a major portion of an inherent bodycapacitance, C_(BODY), of the user.
 3. The training aid stimulator ofclaim 1, wherein the capacitor equivalent is a capacitor.
 4. Thetraining aid stimulator of claim 3, wherein the discharge switch moduleis arranged in an electrical coupling between the capacitor and any skinelectrode effectively isolating the body of the user from the capacitor,when not discharging.
 5. The training aid stimulator according to claim1, further comprising a measurement switch for connecting anddisconnecting the voltage measurement module, wherein the processorfurther is configured to repeatedly switch on and switch off themeasurement switch.
 6. The training aid stimulator according to claim 1,wherein the stimulator is configured to provide a discharge signalcomprising one pulse by closing and opening the discharge switch moduleonce.
 7. The training aid stimulator according to claim 1, wherein thetraining aid stimulator is configured to provide a discharge signalcomprising two or more pulses by closing and opening the dischargeswitch module repeatedly.
 8. The training aid stimulator according toclaim 1, wherein the training aid stimulator being configured to providea discharge signal comprising two, three or four pulses of 1-10milliseconds length and 5-20 milliseconds periods of zero voltage inbetween.
 9. The training aid stimulator according to claim 1, whereinthe body impedance measurement module is arranged electrically such thatthe charging module can be used to provide a test voltage for themeasurement of the body resistance of the user.
 10. A method for keepinga training aid stimulator prepared to deliver a biofeedback electricalstimulus to a body of a user via body electrodes, the method comprising:applying a charging voltage to a capacitor equivalent; measuring, usinga voltage measurement module, a voltage over the capacitor equivalent,the voltage corresponding to a charging level; disconnecting the voltagemeasurement module when a first voltage level is reached; waiting apredetermined period of time; re-connecting the voltage measurementmodule, and providing maintenance charging if a measured voltage levelis at or below a second voltage level; measuring a body resistance ofthe user using the training aid stimulator; and adapting a voltage of astimulus signal based on the measured body resistance.
 11. A method forproviding a biofeedback electrical stimulus to a body of a user via bodyelectrodes, the method comprising: applying a charging voltage to acapacitor equivalent; measuring, using a voltage measurement module, avoltage over the capacitor equivalent, the voltage corresponding to acharging level; disconnecting the voltage measurement module when afirst voltage level is reached; waiting a predetermined period of time;re-connecting the voltage measurement module, and providing maintenancecharging if a measured voltage level is at or below a second voltagelevel; at proper timing, discharging the capacitor equivalent throughthe body of the user; measuring a body resistance of the user using thetraining aid stimulator; and adapting a voltage of a stimulus signalbased on the measured body resistance.